In many parts of the world, the impact of renewable energy, especially from intermittent sources as wind and solar is continuously increasing. In Germany, the share of renewable energy in electricity production is believed to increase from 32.5% in 2015 to 50% in 2030. In order to operate an electrical system and control the mains frequency, the power supply must match the consumption at any time. Ancillary services like primary and secondary control are used to balance the system on a time-scale of several seconds up to 15 minutes. Those control reserves are usually provided by thermal power plants. Particularly in times of high shares of fluctuating renewable feed-in, thermal power plants are turned off or operated at minimum load to avoid electricity production at low electricity prices. However, an amount of about 3000 MW of fast responding primary control need to be provided in the European network of transmission system operators for electricity grid to maintain stable operation even in case of two simultaneous large unit outages. This requirement leads to situations, where thermal power plants are operated in minimum load below their marginal cost to provide control reserves even if there is a surplus of energy in the grid. Operation in low load while at the same time providing control reserves leads to new challenges. As the relation between energy production and the thermal storage capacities provided by the metal and fluid mass in the boiler is decreasing with the load, the ability of responding to control demands is naturally slowed down. Dynamic simulation of the thermodynamic power plant process turned out to be an efficient method to investigate such operational modes. Using comprehensive process models coupled with a control system model, equipment adaptions or control system updates can be evaluated in order to provide faster responses. By increasing the specific amount of ancillary services per unit, the number of units necessary to provide the total amount of primary and secondary control could be reduced in situations with energy surplus.
In order to meet future demands, existing and new plants need to be optimized to offer additional control reserves to stabilize an electrical grid, which is highly penetrated by fluctuating renewables. Since this requires a dynamic investigation, transient physical based models of different power plants have been developed to evaluate effects of increased flexibility as well as to develop optimization strategies. The approach has been tested for a specific 500 MW lignite-fired power plant. It includes detailed modeling of the incorporated sub-systems and their interactions as well as the implementation of the power plant’s control system. The dynamic simulation model is used for the identification of energy storage potentials within the process and for testing and developing control strategies in order to increase flexibility and marketable output of the process. The strategies are benchmarked and evaluated based on the consideration of exergetic efficiency and lifetime-consumption of critical components.
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